Despite therapeutic advances over the last decades, the 5-year overall survival of lung cancer patients remains dismal. In an important breakthrough, recent studies have shown efficacy of immunotherapy in the treatment of lung cancer and other malignancies. PD-1 checkpoint blockade is an especially promising approach, yet response rates remain relatively low (~20% in lung cancer); thus, new approaches are needed to enhance efficacy. Notably, benefit from immunotherapy, including T cell checkpoint blockade, is often associated with elevated pre-treatment expression of immuno-stimulatory genes in tumors, especially T cell chemokines including CCL5, CXCL9 and CXCL10. We hypothesized that an unbiased screen to identify FDA-approved oncology agents with immuno-stimulatory properties would identify agents that augment the response to immunotherapy. In our cancer cell-based screens designed to identify small molecules that induce the expression of T cell chemokines, we found that only a single agent class, HDAC inhibitors (HDACi), induced expression of these chemokines in an array of mouse and human lung cancer cells. Focusing on the HDACi romidepsin, we found that this agent induced a strong anti-tumor response against KRAS mutant non-small cell lung cancer tumors in mice, and that this was entirely dependent on the presence of T cells. Importantly, romidepsin co-treatment markedly augmented the response to PD-1 blockade. These results support using HDACi in combination with PD-1 blockade as a new approach for the treatment of lung cancer patients. However, key in vivo mechanisms of action of HDACi and of synergy with anti-PD-1 need to be defined, especially the effect of these agents on tumor cells and tumor-infiltrating T cells. These studies will rigorously test the hypothesis that targeting HDACs provides a new strategy for generating a tumor microenvironment favorable for checkpoint blockade immunotherapy. Three Specific Aims will test this hypothesis. Aim 1: Defining in vivo mechanisms of chemokine expression and T cell trafficking in NSCLC tumors. Aim 2: Investigating stimulatory effects of HDACi on TIL function and synergism with PD- 1 blockade. Aim 3: Efficacy of novel combinatory strategies to augment PD-1 blockade response in KRAS/TP53 and KRAS/STK11 autochthonous lung tumor models.